This invention relates to a novel process for preparing ascorbic acid. L-ascorbic acid, or Vitamin C, is required in the human diet and is widely sold in tablet form and as an additive in various foodstuffs to meet this need and also as an antioxidant. In all animals except primates and guinea pigs L-ascorbic acid is biosynthesized from D-glucose. The final step in this biosynthesis is the enzymatic conversion of L-gulono-1,4-lactone to L-ascorbic acid. British Pat. No. 763,055 discloses the conversion of L-gulono-1,4-lactone to L-ascorbic acid in about 40% yield by the use of an enzymatic oxidation system.
Attempts to effect the direct conversion of L-gulono-1,4-lactone to L-ascorbic acid by chemical means have been only partly successful, since over-oxidation and degradation reactions produce undesirable by-products. However, low yields of L-ascorbic acid have been produced. For example, Berends and Konings, Rec. Trav. Chim. des Pays-Bas, 74, 1365 (1955) disclose the use of Fentons reagent to give about a 10% yield of L-ascorbic acid. The most successful and common method of producing L-ascorbic acid is based on a multi-step synthesis from D-glucose going through sorbose and 2-keto-gulonic acid as intermediates. Many improvements in the original sorbose method of Reichstein and Grussner, Helv. Chim. Acta., 17, 311 (1934) have been made. D-ascorbic acid may be used as an antioxidant in foodstuffs.
Derivatives of L-gulono-1,4-lactone are known in the art. For example, Matsui et al. have prepared 2,3:5,6-di-O-isopropylidene-L-gulono-1,4-lactone, 3,5-O-benzylidene-L-gulono-1,4-lactone, 2,6:3,5-di-O-benzylidene-L-gulono-1,4-lactone (Yakugaku Zasshi 86, 110 (1966)), and 2,3,5,6-tetrabenzoyl-L-gulono-1,4-lactone has been prepared by Kohn et al. (J.A.C.S., 87, 5475 (1965)). Similar compounds derived from D-gulono-1,4-lactone are also known including 2,3:5,6-di-O-isopropylidene-D-gulono-1,4-lactone, 2,3-O-isopropylidene-D-gulono-1,4-lactone and 5,6-O-isopropylidene-D-gulono-1,4-lactone (Hulyalkar et al., Can. J. Chem., 41, 1898 (1963)). Other compounds include 2,3,5,6-tetra-O-trimethylsilyl-D-gulono-1,4-lactone (Meguro et al., Agr. Bio. Chem., 36, 2075 (1972)), 2,3,5,6-tetra-O-acetyl-D-gulono-1,4-lactone (Ness et al., J.A.C.S., 73, 4759, (1951)) and 2,3,5,6-tetra-O-benzoyl-D-gulono-1,4-lactone (Kohn et al., J.A.C.S., 86, 1457 (1964)). Similar derivatives of gulonic acid and gulonamide have been prepared. Similar derivatives of the other 1,4-lactones are also known. Prior to the present process, the oxidation of partially protected 1,4 lactones is not known to have been described nor are such compounds known to have been employed as intermediates in the preparation of ascorbic acid.